A vehicle has parking sensors to detect an obstacle behind the vehicle. The parking sensors determine a distance of the vehicle from the obstacle using ultrasonic signals when backing a vehicle. The parking sensor operates at ultrasonic frequencies. The parking sensor outputs an ultrasonic detecting signal to detect whether any obstacle is behind the rear of the vehicle and receives an ultrasonic signal as reply from the obstacle. A vehicle generally requires multiple parking sensors to cover the entire rear of the vehicle which makes it a cost intensive solution. Also, the ultrasonic parking sensors use a time division obstacle detecting method in which each sensor sends and receives ultrasonic detect signal in a defined time slot. Thus, the process of detecting obstacles using ultrasonic sensors is time consuming which is unsafe in vehicles moving with high velocity.
Ultrasonic parking sensors require the measurement and drilling of holes in the vehicle's bumper to install transducers. There are risks associated with drilling and mounting the transducers into the bumper. The performance of the Ultrasonic sensors is sensitive to temperature and atmospheric conditions such as snow and rain. The performance of ultrasonic sensors is severely degraded when the sensors are covered with snow. In addition, the range over which the ultrasonic sensors operates is limited.
The use of radars in automotive applications is evolving rapidly. Radars do not have the drawbacks discussed above in the context of ultrasonic sensors. Radar finds use in number of applications associated with a vehicle such as collision warning, blind spot warning, lane change assist, parking assist and rear collision warning. Pulse radar and FMCW (Frequency Modulation Continuous Wave) radar are predominantly used in such applications. In the pulse radar, a signal in the shape of a pulse is transmitted from the radar at fixed intervals. The transmitted pulse is scattered by the obstacle. The scattered pulse is received by the radar and the time between the transmission of the pulse and receiving the scattered pulse is proportional to a distance of the radar from the target. For better range resolution, a narrower pulse is used which requires a high sampling rate in an ADC (analog to digital converter) used in the pulse radar. In addition, sensitivity of a pulse radar is directly proportional to the power which complicates the design process of the pulse radar.
In an FMCW radar, a transmit signal is frequency modulated to generate a transmit chirp. An obstacle scatters the transmit chirp. The scattered chirp is received by the FMCW radar. A signal obtained by mixing the transmitted chirp and the received scattered chirp is termed as a beat signal. The frequency of the beat signal is proportional to the distance of the obstacle from the FMCW radar. The beat signal is sampled by an analog to digital converter (ADC). A sampling rate of the ADC is proportional to the maximum frequency of the beat signal and the frequency of the beat signal is proportional to the range of the farthest obstacle which can be detected by the FMCW radar. Thus, unlike in the pulse radar, the sampling rate of the ADC in the FMCW radar is independent of the range resolution.
Typically in an FMCW radar, multiple chirps are transmitted in a unit called as frame. A 2-dimensional (2D) FFT is performed on the sampled beat signal data received over a frame for range and relative velocity estimation of the obstacle. A bin is a 2D FFT grid that corresponds to a range and relative velocity estimate of an obstacle. A signal detected in a specific bin represents the presence of an obstacle with a predefined range and relative velocity. When multiple receive antennas are used to receive the scattered chirp, the FMCW radar estimates an elevation angle of the obstacle and an azimuth angle of the obstacle. In each frame, a 2D FFT is computed using the data received from each receive antenna. Thus, the number of 2D FFT's is equal to the number of the receive antennas. When an obstacle is detected in a specific bin of the 2D FFT grid, the value of the specific bin corresponding to each of the receive antennas is used to estimate the azimuth angle and the elevation angle of the obstacle.
The FMCW radar resolves obstacles in the dimensions of range, relative velocity and angle. To accurately estimate position of the obstacle, it is required that the obstacle is resolved in any one of these dimensions. Thus, if there are multiple obstacles at the same distance from the FMCW radar and travelling with same relative velocity, the FMCW radar is required to resolve these obstacles in angle dimension. Thus, angle estimation is an important factor in determining the performance of the FMCW radar. The resolution and accuracy of the angle estimation is directly proportional to the number of antennas unit in the FMCW radar. As FMCW radars are used in a broad range of applications, their design becomes more cost-sensitive. Each antenna used to receive the scattered chirp has a distinct receiver path which includes amplifiers, mixers, ADCs and filters. Thus, the number of antennas used in the FMCW radar is a key factor in determining the overall cost of the FMCW radar. Therefore it is important to minimize the number of antennas and processing requirements of the FMCW radar and at the same time maintaining optimum performance level and accuracy.